Maytansinoids are potent antimitotic agents that interfere with the formation of microtubules through the inhibition of the assembly of tubulin (Remillard, et al. (1975) Science 189: 1002-1005). It has been shown that maytansinoids are 100 to 1000 fold more cytotoxic than conventional cancer chemotherapeutic agents like methotrexate, daunorubicin, and vincristine (U.S. Pat. No. 3,896,111). However, use of maytansinoids in human clinical trials was intolerable because they displayed an inadequate therapeutic window in vivo. A much wider therapeutic window can be obtained when maytansinoids are conjugated with antibodies that target specific cells to form antibody-drug conjugates (ADCs). Due to their high potency, maytansinoids are ideal candidates as payloads of ADCs. It was found that C-3 esters with derivatives of N-methyl-L-alanine were much more cytotoxic than the corresponding esters of simple carboxylic acid and 100 times more cytotoxic than their epimers corresponding to N-methyl-D-alanine (U.S. Pat. Nos. 4,137,230; 4,260,608; Kawai, et al. (1984) Chem. Pharm. Bull. 32: 3441-3451; Widdison, et al. (2006) J. Med. Chem. 49: 4392-4408).
The current method for direct coupling of maytansinol with a carboxylic acid by DCC/ZnCl2 has the drawbacks of low yields and poor reproducibility. Further, when chiral amino acid derivatives such as N-methyl-L-alanine derivatives are coupled with maytansinol, complete epimerization of side chain chiral center occurred giving L and D-aminoacyl ester in a 1:1 ratio. This greatly reduced the yield of the desired product in the total coupled products. Furthermore, tedious chromatographic separation (both column and HPLC) of the desired product from the unwanted diastereomer was needed. (U.S. Pat. Nos. 4,137,230; 4,260,608; 5,208,020; 5,416,064; 6,333,410; Kawai, et al. (1984) Chem. Pharm. Bull. 32: 3441-3451; Widdison, et al. (2006) J. Med. Chem. 49: 4392-4408).
Another method for preparation of maytansinol C-3 ester is through forming an anion of maytansinol with a suitable base such as metal hexamethyldisilazide (metal=Li, Na, K, Zn) followed by reacting the anion with an activated carboxy compounds such as anhydride and acyl fluoride. Additional reaction steps are needed for conversion of carboxylic acid to its anhydride or acyl fluoride, which are typically unstable intermediates. Because of high water and air sensitivity of metal hexamethyldisilazide, strict water free and oxygen free conditions under low temperatures are required. Furthermore, large excess of expensive zinc hexamethyldisilazide is required to generate the coupling product in a good yield. While US Patent Application Publication No. 2006/0167245 asserts that little epimerization of chiral center occurred, no reaction yield was given.
Because the starting material maytansinol is very expensive and the existing methods for coupling maytansinol with carboxylic acid resulted in low yield and/or epimerization of side chain chiral center, a method for preparation of maytansinoid esters with high yield and high diastereoselectivity by direct coupling of maytansinol with a carboxylic acid, especially a chiral amino acid such as N-methyl-L-alanine derivative, is greatly needed to speed up the research and development process of cell binding agent maytansinoid conjugates for target cancer therapy.